Plasma channel drilling process

ABSTRACT

Material is removed from a body of material, e.g. to create a bore hole, by plasma channel drilling. High voltage, high energy, rapid rise time electrical pulses are delivered many times per second to an electrode assembly in contact with the material body to generate therein elongate plasma channels which expand rapidly following electrical breakdown of the material causing the material to fracture and fragment.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Section 371 National Stage Application ofInternational Application No. PCT/GB03/00622 filed on Feb. 12, 2003.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a method and apparatus for removal ofmaterial from a body of material and, in particular but not exclusively,to a method and apparatus for removal of material to form a bore duringdrilling operations.

2) Description of Related Art

Currently known methods of plasma or electric arc drilling rely on theprocess of heating the material to be removed to its melting point. Thisprocess requires a significant amount of electrical power, and has thefurther difficulty of accurately focussing the power source on thematerial, or formation, which is to be removed. This may, therefore,result in the production of wide “kerfs” being formed during the processof producing a bore, as well as the waste of electrical power.

Another known method is spark drilling which utilises anelectro-hydraulic shockwave, initiated by a plasma discharge, whichpropagates through a fluid medium and impinges on the material causingthe material to disintegrate or fracture. The major difficulty with thisprocess is focussing the shockwave onto the material. If the shockwaveis not focussed much of the shockwave energy is wasted resulting in apoor “drilling” rate. In addition, the resulting hole is not a clearlydefined, circular borehole.

It is an object of the present invention to obviate or mitigate at leastone of the problems associated with the prior art.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan apparatus adapted for removal of material from a body of material,the apparatus comprising a high voltage pulse generator coupled to anelectrode assembly located at a material-removal station of saidapparatus, said apparatus being adapted to generate electrical pulses tocreate a plasma channel repetitively per second within or on the surfaceof the body of material and material is removed by the rapid expansionof each plasma channel fracturing and fragmenting the body of material.

The electrical breakdown of a solid body of material by plasma-channeldrilling in accordance with the present invention results in theformation of an electrically conductive gas-plasma-filled breakdownchannel in or on the solid body. The electrical resistance of thisplasma-filled breakdown channel correlates with the physical propertiesof the solid body and depends on characteristics of the dischargecircuit of the pulsed generator and the electrode assembly (initialpotential of output, capacitance of the circuit, inter-electrodespacing). As the discharge channel expands during the first hundreds ofnanoseconds after the electrical breakdown of the material, the diameterof the discharge channel increases from several micrometers to severalhundred micrometres, so as to accommodate the high current from thedriving circuit. This expansion of the discharge channel acts like apiston on the surrounding material, causing the material to fracture andfragment.

By virtue of the present invention the plasma channel, which isgenerally elongate, is formed many times per second with differentangular (radial) orientation and an effective and controlled drillingaction is achieved without the requirement to heat the rock formation orbody of material to its melting point or to focus a shockwave onto thesurface of the formation or body of material or to have a rotary drillbit.

Preferably, the high voltage pulsed generator is capable of producinghigh peak powers in the range 1-100 MW. Peak powers which have beenfound to be effective in particular tests are 5, 8 and 12 MW.

Preferably, the high voltage pulsed generator includes a drive circuitto enable high voltage pulses in the range 10-50 kV to be produced. Theapplied voltage pulses may be of a positive or negative polarity.Positive polarity voltage pulses are preferred because the plasmachannel extinguishes more rapidly than with the negative polarityvoltage pulses.

Preferably, the generator is capable of producing high voltage pulseshaving an energy level in the range of 10-500 Joules.

The drive circuit may allow a pulse repetition rate in the range 1-100pulses per second for the high voltage pulses to be produced, although arepetition rate of between 5 and 25 pulses per second is typical.

Preferably, the pulse created by the pulse generator is in the form ofan impulse and has a duration in the range 1-50 microseconds with a risetime less than 150 nanoseconds. A pulse rise time of the order of 100nanoseconds is preferred.

One factor that limits the effectiveness of the pulse repetition rate inremoval of material, is the time it takes for the plasma channel todeplete to a sufficient level before the creation of a next plasmachannel can have full effect in removal of the material. Therefore,preferably, the pulse generator produces a high voltage pulse which hasa current waveform close to that of the critically damped response ofthe circuit created when the plasma channel is produced.

The electrode assembly comprises at least one electrode pair formed bytwo electrodes between which the plasma channel is formed: a firstelectrode being a high voltage electrode, and a second electrode being areturn.

Preferably, the two electrodes are co-axially arranged.

Advantageously the electrodes are arranged such that they may contactthe surface of the material to be removed.

The electrode assembly may be provided in a number of shapes and sizes.In one embodiment the electrode assembly includes an internal discshaped electrode and an external cup-like electrode, with an annularinter-electrode gap.

The apparatus may be provided with means for removal of waste materialfrom the cutting surface. The outer electrode may be adapted to allow afluid to be transferred to the cutting surface so as to remove the wastematerial. The outer electrode may be further adapted to allow the fluidto be circulated around or through said outer electrode. The fluidpreferably has the dual function of removing waste material andfunctioning as a cutting/lubricating fluid. For this latter purpose itis preferred that the fluid is of low conductivity, such as tap water ormineral oil.

The outer electrode maybe of the form of a cylindrical cage-likestructure that is provided with suitable fluid ingress holes, andapertures or slots to allow fluid-entrained removed-material to escape,when using the apparatus to produce a bore in the material.

The sizes for the various components of the electrode assembly will varydepending on the size of bore to be created, however, for an electrodeassembly having an external electrode of 50 mm diameter, the centralelectrode disc may be around 30 mm in diameter and the inter-electrodegap spacing about 7 mm, with the external cup electrode being around 3mm thick.

Preferably, the inter-electrode gap spacing together with the parametersof the electrical drive circuit are optimised in conjunction with thephysical properties of the material to be removed, such that the currentwaveform produced is close to the critically damped response.

For drilling in hard rock conditions it has been found that when the endface of the outer electrode is sharpened a better drilling rate isachieved in comparison with a non sharpened electrode. In test boreholes in samples of hard sandstone the sharpened drill assemblydemonstrated a drilling rate of 2.5 cm/min in comparison with a drillingrate of 0.6 cm/min for the non-sharpened outer electrode. In test boreholes drilled in soft sandstone both drill heads demonstratedapproximately equal drilling rates.

In one embodiment the electrode assembly may be in the form of a sondeand the high voltage pulse generator an electrical cartridge. Thisconfiguration enables the sonde and cartridge to be deployed down-holein a well or bore by means of coiled tubing, drill pipe, wireline or thelike.

The electrical cartridge may be adapted such that it may be coupled to acable, thereby enabling the electrical cartridge to be powered by asurface-located power source.

Alternatively, the electrical cartridge may be adapted such that it maybe powered by a down-hole power source such as a mud-driven generator.

According to a second aspect of the present invention, there is provideda method of removing material from a body of material, the methodcomprising the steps of providing a high voltage pulsed generatorcoupled to an electrode assembly, and repeatedly per second generatingelectrical pulses which create a plasma channel within or on the surfaceof the body of material, thereby causing material to fracture andfragment from the body due to the rapid expansion of each plasma channelfollowing electrical breakdown of the material.

Preferably, the method includes the step of contacting the electrodes ofthe electrode assembly with the surface of the body of the material.

Preferably the method further includes the step of removal of wastematerial fractured and fragmented from the body of material.

In one embodiment the method also includes the step of ensuring the bodyof material, or at least a surface portion of said body of material, isimmersed in a liquid, such as water. Alternatively the body of materialmay be dry.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other aspects of the present invention will become apparentfrom the following description when taken in combination with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a plasma channel drilling apparatusaccording to the present invention;

FIG. 2 is a cross-section view of the electrode assembly of FIG. 1;

FIG. 3 is a schematic diagram of the plasma channel drilling process;

FIG. 4 is a schematic diagram of a pulse generator circuit;

FIG. 5 illustrates typical voltage and current waveforms duringplasma-channel formation;

FIG. 6 is a graph showing breakdown delay time-voltage characteristicsof sandstone and water for various inter-electrode distances;

FIG. 7 is a graph showing breakdown delay time-electric fieldcharacteristics of water, transformer oil, quazite, porphirite, marble,shale and sandstone;

FIG. 8 is a graph of pre-breakdown delay time for water saturatedsandstone at different inter-electrode distances.

DETAILED DESCRIPTION OF THE INVENTION

Plasma-channel drilling, as explained herein, is the process ofdelivering electrical power to an electrode assembly acting as a drillbit, in a sequence of discrete high power pulses to form highlydestructive short lived electrical plasma channel discharges, which inturn cause localised fragmentation and disintegration of a material,such as a rock structure, ahead of the electrode assembly.

Referring to FIG. 1, there is illustrated a plasma-channel drillingapparatus, generally represented by reference numeral 10, for removal ofsurface material 12 from a body of material 16. The apparatus 10comprises a high voltage pulsed generator 18 which is coupled by an HVcable 9 to an electrode assembly 20. The electrode assembly 20 isarranged such that a plasma channel 22 is produced within or on thesurface of the body of material 16, which causes a localised region ofthe surface material 12 to fracture and fragment.

The high voltage pulsed generator 18 includes a drive circuit capable ofproducing high voltage pulses, between 10-50 kV at an energy level ofabout 10-500 Joules per pulse. The drive circuit also enables a pulserepetition rate of 1-100 pulses per second to be produced at theelectrode assembly 20, thereby forming a plasma channel at up to 100times per second resulting in an effective and controlled drillingprocess.

An increase in the pulse repetition rate of the HV generator 18 does notnecessarily result in an increase in the drilling rate of the plasmachannel drilling apparatus 10. In one test bore holes drilled using adrive circuit set to 35 kV at an energy level of 122.5 Joules per pulse,resulted in a drilling rate of 5 cm per minute at a pulse repetitionrate of 10 pulses per second. However, in a different sample a drillingrate of 6.5-7.5 cm per minute was achieved with a pulse repetition rateof 5 pulses per second.

Furthermore, the pulse repetition rate has a direct effect on the energyefficiency of the apparatus, with a decrease in the pulse repetitionrate resulting in a decrease in the specific energy consumption. Thus a20% decrease in the specific energy consumption was achieved at a pulserepetition rate of 5 pulses per second compared to that of 10 pulses persecond, when drilling with apparatus set at an energy level of 207.5Joules per pulse, 38.5 KV and output generator capacitance of 280 nF.

The pulse repetition rate as regards removal of material is related tothe time taken for the created plasma-channel to deplete to a sufficientlevel, before a succeeding generated plasma channel can have full effecton removal of material. This is due to the fact that the plasma channelcauses the material to fracture and fragment by rapid expansion of theplasma channel on or within the surface of the material, and it istherefore necessary to wait until the created plasma channel hassubsided sufficiently before the next plasma channel is created.

An increase in the pulse energy of the HV generator has a direct effecton the specific energy consumption of the plasma channel drillingprocess. The increase of the energy available per pulse results inimprovement of the energy efficiency of the plasma drilling channelapparatus. In test bore holes drilled in sandstone samples using a drivecircuit set to 35 kV at an energy level of 61 J/pulse resulted in aspecific energy of drilling of 803 J/cm³ at a pulse repetition frequencyof 10 pulses per second, and in a specific energy of drilling of 474J/cm³ at a pulse repetition frequency of 5 pulses per second at anenergy level of 122.5 J/pulse. Increasing the energy available per pulseby a factor of 2 for a constant voltage resulted in a 59% reduction inthe specific energy of drilling. Further increase of the energy wouldresult in saturation and a consequent decrease in the efficiency of thedrilling apparatus. Therefore for maximising of the efficiency of theplasma channel drilling apparatus it is necessary to determine theoptimal parameters (particularly applied voltage, pulse repetitionfrequency, energy available per pulse) for different materials to whichthe plasma-channel drilling apparatus is being applied.

Referring to FIG. 2, there is shown a cross section of the electrodeassembly 20 of FIG. 1. The electrode assembly 20 comprises a HVelectrode 32 which is made from a material such as stainless steel inorder to increase the lifetime and reliability of the overall assembly.The HV electrode 32 is coupled to an HV shank 34 by a threaded andpinned portion 36 so as to secure the electrode 32 in position. The HVshank 34 is in turn coupled via a connector 40 to the core 38 of HVcable 9 connecting pulse generator 18 to the electrode assembly 20.

The connector 40 is arranged such that at one end of the connector 40there is provided a threaded portion 58 into which the HV shank 34 iscoupled, and at an opposite end there is provided a bore 60 into whichthe cable core 38 is fitted. The HV cable core 38 is secured in placevia grub screws 62.

Surrounding the shank 34, connector 40 and cable core 38 are plasticinsulators 42, 43, 44 which prevent electrical breakdown from occurringbetween the HV components of the electrode assembly and the return, orgrounded portions of the electrode assembly.

A cup-shaped grounded electrode 46 surrounds the HV electrode 32, therebeing a predetermined inter-electrode gap spacing 48 between the twoelectrodes 32, 46. Electrode 46 at its exposed annular end lies in thesame plane as the exposed outer surface of the disc electrode 32 (andmay have a sharpened end face or edge). The grounded electrode 46 iselectrically connected to a grounded metal tube or pipe 50, such ascopper, via a conductive sleeve 52, which has male and female threadedportions, to which the metal pipe 50 and grounded electrode 46 arerespectively connected. The metal pipe 50 is electrically connected atan opposite end, via cable 9 to the pulse generator 18, creating areturn path for the flow of current to pass when the plasma channel 22is created. The upper portion of the grounded electrode 46 is providedwith slots 70 and with through holes 54 to which are connected pipes 72allowing a fluid such as water to be passed to the inter-electrode gap48.

As is schematically shown in FIG. 3 this arrangement enables a flow offluid such as water to be supplied to cutting surface 66 to entrain andremove cuttings or debris 68 by circulating water down through thecopper pipes 72 and up into the drilled bore 74, such that the waterwill pick up and carry away the cuttings 68 from the cutting surface 66.The cuttings 68 may be carried away from the cutting surface 66 eithertraversing the gap 48 between the two electrodes, or, without traversingthe gap 48, through venting slots 70 provided in the grounded electrode.

The plasma channel 22 is also shown in FIG. 3 at two of itsinstantaneous positions during a drilling period. Channel 22 at eachtime instant is elongate and extends in an arc from an edge or face ofelectrode 32 to a local region of an inner edge or lower face of theouter electrode 46. FIG. 3 also illustrates the applied electrical pulsein the form of a fast-rise impulse.

Now referring to FIG. 4, the pulsed generator 18 comprises (typically)an energy storage capacitor 78 which is charged by a primary powersource 80 at a voltage level up to 50 kV, via a coupling resistor 82(e.g. 100 ohms) and a wavetail or decoupling resistor 84 (e.g. 10kilo-ohms) with switch 86 open. When the capacitor 78 is fully charged,switch 86 is closed on command. The switch closure transfers energy fromthe capacitor into the electrode assembly 20 via the high voltageco-axial feed 9. Energy from the capacitor 78 and the high voltageco-axial feed 9 is dissipated in the plasma-channel with a currentwaveform determined by the natural oscillatory frequency of the circuit.This energy dissipation in the plasma-channel results in the drillingprocess. FIG. 5 illustrates typical current and voltage waveformsgenerated during plasma channel formation.

Thus, on the micro second timescale denoted in FIG. 5, the first voltagepulse commences at about 6 μs and rises very rapidly in far less than 1μs to about 35 kV. The voltage level remains in the range 35 kV droppingto 26 kV over the time interval 6 μs to about 13 μs during which timethe body of material is electrically stressed but without breakdownoccurring. Electrical breakdown occurs at about 13 μs when theconductive plasma channel is formed (and physically expands rapidly) andthe voltage collapses in damped oscillation to terminate at about 40 μswhilst concurrently the plasma channel current is established as adamped oscillation also terminating at about 40 μs.

When the energy in the capacitor 78 has been dissipated in theplasma-channel circuit, the power source 80 recharges the capacitor 78by opening of switch 86, ready for another cycle of the circuit.

Plasma channel creation between the electrodes of the electrode assemblyis dependent upon a number of factors, which include the electrodeprofile, electrical properties of the fluid on or in the material andthe material itself; temperature, pressure, voltage magnitude and pulseprofile. Furthermore, if the apparatus is self-firing, i.e., there is notrigger signal to initiate the creation of the plasma channel, it isimportant to provide an electrode assembly having a pre-characterisedgeometry for the fluid and material present to ensure that the appliedhigh voltage pulse initiates the desired plasma formation. If theelectrode assembly is not configured for the specific environmentalconditions, then a significant amount of the energy available may belost through ionic conduction in the fluid.

It has been found that for fast rising voltages (approximately 10 MV permicrosecond), solids suffer dielectric breakdown (plasma formation)earlier than fluids such as water and oil. Referring to FIG. 6, there isprovided a graph showing breakdown delay time against voltagecharacteristics of sandstone and water for different inter-electrodedistances. It can be seen for these sets of curves, that if the appliedvoltage is high enough the sandstone will suffer electrical breakdown(plasma formation) more rapidly than the water.

FIG. 7 shows the relationship between applied electric field andbreakdown delay time for different rock structures, transformer oil andwater. These sets of curves show the same trend as in FIG. 6, in that ifthe applied voltage is high enough the different rocks will sufferelectrical breakdown more rapidly than water or transformer oil.However, for the transformer oil, it can be seen that the electric fieldapplied to the electrode assembly must be greater than that of water toensure electrical breakdown in the rock structure before that of thetransformer oil. Therefore, for plasma channel drilling it is desirableto apply as high a voltage with as rapid a rise time as possible to theelectrodes, in order that the rock suffers electrical breakdown beforethe water or oil. This would theoretically maximise the efficiency ofthe drilling process, except that for certain applications it isdesirable to restrict the maximum operating voltage of the system toless than 50 kV. Voltages above this value, of 50 kV, result in systeminsulation requirements becoming of significance, which may result in anincrease in the overall size of the drill, electrical feed and powersupply.

FIG. 8 shows the relationship for negative polarity pulses between thedelay time and electrode gap spacing for water-saturated red sandstone.It can be seen that for both applied voltages (−30 kV and −35 kV) thedelay time increases with increasing gap spacing. This is expected sincethe electric field, which influences the delay time, decreases withincreasing gap size. The disadvantage that the delay time has withrespect to plasma channel drilling, is that as the delay time increases,the amount of energy available to the plasma channel is reduced due tolosses through the water.

Plasma channel drilling can be conducted in rock formations saturated inbrine or oil. In spite of the fact that the salinity of connate water inoil bearing rock formation can be as high as 100 g of electrolyte perone liter of water, the use of low conductive water as a drilling fluidsignificantly reduces the delay time and ionic conduction losses andprovides an effective plasma channel drilling process. In test boreholes in sandstone samples the plasma channel drilling apparatusdemonstrated a drilling rate of 7.0 cm/min for water-saturatedsandstone, and 5.5 cm/min for brine-saturated sandstone with the use oftap water as the drilling fluid. In order to produce efficient plasmachannel drilling lower conductivity drilling fluids such as tap water ormineral oil should be used.

Therefore, to maximise the energy available to the plasma channel, thegap spacing must be reduced or the voltage increased in order to reducethe delay time.

As previously stated, plasma-channel drilling uses a plasma dischargethat is formed on the surface or through the material to be drilled.Therefore, in order to produce an efficient plasma channel drillingprocess it is necessary to maximise the rate of pressure rise during thefast expansion period, that is during the first few hundreds ofnanoseconds of creating the plasma channel. This may be achieved bymaximisation of mean power dissipated in the active load of the plasmachannel during the first half period of the current oscillation, whichmay be accomplished by producing a pulse having a duration in the regionof 1-50 microseconds and having a rise time of less than 150nanoseconds, preferably in the order of 100 nanoseconds.

The pulse generator used to drive the drilling process can generate highpeak powers of between 10-100 MW at the electrode assembly. However, dueto known pulsed power and energy consumption techniques, the averagepower output for the generator is in the region of a few kilowattswhilst drilling. This enables the pulse generator and associatedequipment to be compact and portable, such that the apparatus can bedeployed by wire-line or coiled-tubing equipment into a bore, with theplasma channel apparatus split into a downhole electrode assembly and asurface pulse generator. Alternatively, the pulse generator may beincorporated into an electrical cartridge such that the pulse generatorand the electrode assembly may be deployed together within the bore.

By exploiting the differences in temporal dielectric strength betweenthe fluid within the bore and the rock formation, as can be derived fromthe graphs shown in FIGS. 6 and 7, the plasma-channel is forced to formalong the surface of, or inside the formation ahead of the apparatus.

In addition, by utilising single or multiple annular electrodegeometries within the electrode assembly, the plasma-channel will changeposition around the electrode gap so as to seek out new areas ofmaterial, such that different sections of the formation are removed.This is achieved because the plasma-channel seeks out the path of leastresistance, and because the rock formation electrically breaks-downbefore that of the fluid, at high voltages, the plasma-channel will beformed within or on the surface of. the formation. The plasma-channelwill therefore rotate with time through 360° seeking out the path ofleast resistance through the material, thereby removing the materialahead of the electrode assembly and eliminating the requirement torotate the electrode assembly itself.

The electrical breakdown of solids by plasma-channel drilling results inthe formation of a gas plasma filled breakdown conductive channel. Theresistance of this plasma filled breakdown channel is related to theelectrical and physical properties of the channel and depends upon thephysical properties of the solids (ionisation potential, molecularweight) and also depends on characteristics of the discharge circuit(initial potential of output, capacitance of the circuit,inter-electrode spacing).

For optimal performance of the plasma-channel drill, the inter-electrodegap spacing together with the parameters of the electrical drive circuitneed to be optimised, such that the current waveform produced is closeto its critically damped response. In practice, this means that theoptimal inter-electrode gap spacing must be determined for the differentmaterials to which the plasma-channel drilling apparatus is beingapplied.

It is desired to produce a current waveform that is close to itscritically damped response as this has been seen to result in thehighest rate of energy deposition in the plasma breakdown channel.

It will be appreciated that various modifications may be made to theembodiment hereinbefore described without departing from the scope ofthe present invention, e.g., a pulse generator which is deployeddownhole with the electrode assembly may be powered by a downhole powersource, such that there is no need for any surface power to be provided.In this way, the entire apparatus may deployed on drill pipe or the likeand power supplied by the downhole power source. The electrode assemblymay include more than two electrodes, and the cutting removal fluid maybe mud based so as to help balance well conditions.

It will be appreciated that a principal advantage of the presentinvention is that the above apparatus is small, compact and readilydeployable, making the apparatus ideal for work-over applications onplatforms, rigs or the like, to maintain maximum well production.Furthermore, the apparatus can produce small bore holes, typically up to100 mm, that can be exploited to enhance production zones so as toensure maximum productivity from the well.

In addition, this method of drilling produces sub-millimetre drillcuttings in the region of 300 micrometres, in comparison to that ofknown systems in which cuttings in the region of 2-7 millimetres areproduced. The reduction in the size of the cuttings reduces the need touse equipment to further reduce the cutting size, as is common practice,such that the cuttings can be transported via a pipe network to astorage area. In addition, this method of drilling enables the drillcuttings to be readily re-injected into the subsurface formation,thereby reducing the environmental impact, and the amount of wasteproduced.

Other advantages of the invention include the possible reduction influid pump rates; eliminating the need for rotary equipment to drive thedrill bit; and a reduction in the specific energy needed to create abore hole. Initial results have shown that the specific energy forplasma channel drilling is in the region of 250-290 Joules per cm³,compared to 350-560 Joules per cm³ for rotary (oilfield) drilling, of amedium hardness rock.

1. A method of removing material from a body of material comprisingproviding a high voltage pulsed generator coupled to an electrodeassembly, and repeatedly per second generating electrical pulses with aduration in the range 1-50 microseconds and a voltage rise time of lessthan 150 nanoseconds to create a plasma channel within or on the surfaceof the body of material, thereby causing material to fracture andfragment from the body due to the rapid expansion of each plasma channelfollowing electrical breakdown of the material.
 2. A method as claimedin claim 1, wherein the pulse voltage rise time is less than 100nanoseconds.
 3. A method as claimed in claim 1, comprising generatingpulses that have a current waveform close to that of the criticallydamped response of the circuit created when the plasma channel isproduced.
 4. A method as claimed in claim 1, comprising generatingpulses with peak powers in the range 1-100 MW.
 5. A method as claimed inclaim 1, comprising generating high voltage pulses in the range 10-50kV.
 6. A method as claimed in claim 1, comprising generating highvoltage pulses having an energy level in the range of 10-500 Joules. 7.A method as claimed in claim 1, comprising generating pulses at a pulserepetition rate in the range 1-100 pulses per second.
 8. A method asclaimed in claim 7, wherein the pulse repetition rate is between 5 and25 pulses per second.
 9. A method as claimed in claim 1, including thestep of contacting the surface of the body of material with theelectrodes of the electrode assembly.
 10. A method as claimed in claim1, including the step of transporting waste material from the cuttingsurface of the body of material.
 11. A method as claimed in claim 1,including the step of immersing the body of material, or at least asurface portion of said body of material, in a liquid, such as water.12. An apparatus comprising a high voltage pulsed generator coupled toan electrode assembly located at a material-removal station, saidapparatus being adapted to generate repeatedly per second electricalpulses with a duration in the range 1-50 microseconds and a voltage risetime of less than 150 nanoseconds to create a plasma channelrepetitively per second within or on a surface of the body of material,thereby to remove material by the rapid expansion of each plasma channelfracturing and fragmenting the body of material.
 13. An apparatus asclaimed in claim 12, wherein the high voltage pulsed generator isoperable to produce pulses with peak powers in the range 1-100 MW. 14.An apparatus as claimed in claim 12, wherein the high voltage pulsedgenerator is operable to produce high voltage pulses in the range 10-50kV.
 15. An apparatus as claimed in claim 12, wherein the high voltagepulsed generator is operable to produce high voltage pulses having anenergy level in the range of 10-500 Joules.
 16. An apparatus as claimedin claims 12, wherein the generator is operable to generate pulseshaving a pulse repetition rate in the range 1-100 pulses per second. 17.An apparatus as claimed in claim 12, wherein the pulse repetition rateis between 5 and 25 pulses per second and the generator is operable togenerate pulses having a pulse repetition rate in the range 1-100 pulsesper second.
 18. An apparatus as claimed in claim 12, wherein the pulsescreated by the pulse generator are each in the form of an impulse with aduration in the range 1-50 microseconds and a voltage rise time of lessthan 150, but typically less than, 100 nanoseconds.
 19. An apparatus asclaimed in claim 12, wherein the pulse generator produces a high voltagepulse that has a current waveform close to that of the critically dampedresponse of the circuit created when the plasma channel is produced. 20.An apparatus as claimed in claim 12, wherein the electrode assemblycomprises co-axially arranged electrodes, one being an internal discshaped electrode and the other being an outer cylindrical cage-likeelectrode, with an annular inter-electrode gap.
 21. An apparatus asclaimed in claim 12, including means for transporting waste materialfrom the cutting surface.
 22. An apparatus as claimed in claim 12,including means for transporting waste material from the cutting surfaceand wherein the outer electrode is adapted to allow a fluid to betransferred to the cutting surface so as to entrain and transport wastematerial, and to allow the fluid and entrained waste material to becirculated through said outer electrode.
 23. An apparatus as claimed inclaim 12, wherein the electrode assembly is in the form of a sonde andthe high voltage pulsed generator an electrical cartridge, therebyenabling the sonde and cartridge to be deployed down-hole in a well orbore by means of coiled tubing, drill pipe, wireline or the like.
 24. Amethod of removing material from a body of material comprising providinga high voltage pulsed generator coupled to an electrode assembly, andrepeatedly per second generating electrical pulses having a voltage inthe range 10-50 kV to create a plasma channel within or on the surfaceof the body of material, thereby causing material to fracture andfragment from the body due to the rapid expansion of each plasma channelfollowing electrical breakdown of the material.
 25. A method as claimedin claim 24, wherein each pulse has a duration in the range 1-50microseconds and a voltage rise time of less than 150 nanoseconds.
 26. Amethod as claimed in claim 24, wherein each pulse has a duration in therange 1-50 microseconds and the pulse voltage rise time is less than 100nanoseconds.
 27. A method as claimed in claim 24, comprising generatingpulses that have a current waveform close to that of the criticallydamped response of the circuit created when the plasma channel isproduced.
 28. A method as claimed claim 24, comprising generating pulseswith peak powers in the range 1-100 MW.
 29. A method as claimed in claim24, comprising generating high voltage pulses having an energy level inthe range of 10-500 Joules.
 30. A method as claimed in claim 24,comprising generating pulses at a pulse repetition rate in the range1-100 pulses per second.
 31. A method as claimed in claim 24, comprisinggenerating pulses at a pulse repetition rate in the range 5 and 25pulses per second.
 32. A method as claimed in claim 24, including thestep of contacting the surface of the body of material with theelectrodes of the electrode assembly.
 33. A method as claimed in claim24, including the step of transporting waste material from the cuttingsurface of the body of material.
 34. A method as claimed in claim 24,including the step of immersing the body of material, or at least asurface portion of said body of material, in a liquid, such as water.35. An apparatus for removal of material from a body of material, theapparatus comprising a high voltage pulsed generator coupled to anelectrode assembly located at a material-removal station, said apparatusbeing adapted to generate electrical pulses in the range 10-50 kV tocreate a plasma channel repetitively per second within or on a surfaceof the body of material and material is removed by the rapid expansionof each plasma channel fracturing and fragmenting the body of material.36. An apparatus as claimed in claim 35, wherein the electrode assemblycomprises co-axially arranged electrodes, one being an internal discshaped electrode and the other being an outer cylindrical cage-likeelectrode, with an annular inter-electrode gap.
 37. An apparatus asclaimed in claim 35, wherein the electrode assembly is in the form of asonde and the high voltage pulsed generator an electrical cartridge,thereby enabling the sonde and cartridge to be deployed down-hole in awell or bore by means of coiled tubing, drill pipe, wireline or thelike.
 38. A method of removing material from a body of materialcomprising providing a high voltage pulsed generator coupled to anelectrode assembly, and repeatedly per second generating electricalpulses having an energy level in the range of 10-500 Joules to create aplasma channel within or on the surface of the body of material, therebycausing material to fracture and fragment from the body due to the rapidexpansion of each plasma channel following electrical breakdown of thematerial.
 39. An apparatus for removal of material from a body ofmaterial, the apparatus comprising a high voltage pulsed generatorcoupled to an electrode assembly located at a material-removal stationof said apparatus, said apparatus being adapted to generate electricalpulses to create a plasma channel repetitively per second within or on asurface of the body of material and material is removed by the rapidexpansion of each plasma channel fracturing and fragmenting the body ofmaterial, wherein the pulse generator produces a high voltage pulsewhich has a current waveform close to that of the critically dampedresponse of the circuit created when the plasma channel is produced. 40.An apparatus for removal of material from a body of material, theapparatus comprising a high voltage pulsed generator coupled to anelectrode assembly located at a material-removal station of saidapparatus, said apparatus being adapted to generate electrical pulses tocreate a plasma channel repetitively per second within or on a surfaceof the body of material and material is removed by the rapid expansionof each plasma channel fracturing and fragmenting the body of material,wherein the electrode assembly is in the form of a sonde and the highvoltage pulsed generator an electrical cartridge, thereby enabling thesonde and cartridge to be deployed down-hole in a well or bore by meansof coiled tubing, drill pipe, wireline or the like.